swift234

Sema: Add @availability attributes when synthesizing materializeForSet accessors.

Synthesize implicit @availability attributes to make sure that a synthesized
materializeForSet accessor is available enough to access the underlying storage and its
getter and setter.

These synthesized attributes could trigger redundant diagnostics when a subclass gives
overriding getters or setters non-contravariant availability. We detect when this
happens and suppress the redundant diagnostics.

This commit also improves availability diagnostics in synthesized code. We now respect
synthesized @availability annotations on containing DeclContexts when determining the
potential OS versions that could be executed at invalid SourceLocations.

Swift SVN r27002